A new technique has been developed using a Nd-YAG laser to generate singlet oxygen (1 Delta g) unambiguously, in quantity and at a known rate. Using this technique a number of problems of biological importance will be examined. These include the reactions of singlet oxygen with bilirubin, which is important in the phototherapy of hyperbilirubemia, the protective mechanisms inferred for the carotenoids and the tocopherols, the latter being connected to vitamin E activity and in the reactions of amino acids and nucleic bases in connection with the processes of photodynamic action.